1. Field of the Invention
The invention relates to a vacuum booster enclosure including a pair of shells coupled together, and more particularly, to an improvement of the structure which couples the pair of shells together.
2. Description of the Prior Art
A booster such as a brake booster, a clutch booster or the like generally comprises a pair of shells which are coupled together to define a shell structure. Referring to FIG. 1 which shows an example of the structure which couples the shells together, there is shown a brake booster 1 including a front or a first shell 2 and a rear or a second shell 3. A diaphragm 4 which is to be received within the shell structure has its periphery held between the first and the second shells 2, 3 as the latter are coupled together.
The first shell 2 is generally cylindrical in configuration and is closed at one end. Specifically, adjacent its open end, it includes a cylindrical portion 5 which is formed with a step or rim 6 so that the inner diameter of the cylindrical portion 5 at its open end is slightly greater than the inner diameter of a portion thereof which is inwardly spaced from the open end thereof. As shown clearly in FIGS. 2A and 2B, a plurality of circumferentially spaced claws 7 project radially inwardly along the inner periphery of the opening of the first shell 2.
On the other hand, the second shell 3 is generally disc-shaped, and includes a peripheral portion in which an annular groove 8 which is open in the radially outward direction is formed for receiving the peripheral edge of the diaphragm 4. The diameter of the second shell 3, as measured across the radially extending peripheral edges, coincides with the inner diameter of the outermost enlarged portion of the first shell 2 which is located radially outwardly of the rim 6. A plurality of notches 9, which are equal in number to the number of the claws 7, are formed in the peripheral edge of the second shell 3 in circumferentially spaced apart relationship so as to avoid an interference with the claws 7 when the shells 2, 3 are fitted together.
To couple the first and the second shells 2, 3 together, the peripheral edge of the diaphragm 4 is initially fitted into the annular groove 8 formed in the second shell 3. Subsequently, after the notches 9 formed in the second shell 3 are circumferentially aligned with the claws 7 formed on the first shell 2, the second shell 3 is fitted into the first shell 2. When the second shell 3 has entered the first shell 2, the shells 2, 3 are rotated relative to each other about their axes. Thereupon, the first shell 2 holds the second shell 3 between the claws 7 and the rim 6 while simultaneously holding the peripheral edge of the diaphragm 4. In this manner, the resilience of the diaphragm 4 permits the shells 2, 3 to be locked against rotation.
In the conventional booster 1 mentioned above, the clamping action between the shells 2, 3 solely depends upon the resilience of the diaphragm. Accordingly, both shells must be shaped to a high accuracy to ensure that these shells cannot be rotated relative to each other. This presents a difficulty with respect to quality control, thus preventing a reduction in the manufacturing cost from being achieved.